During technology development many parameters which influence process performance are not yet known. This can include layout information such as design rules, cell libraries, and optical parameters. The development may begin with a shrunken version of a cell library of the previous technology. The question then arises as to how this layout is transformed into silicon in the new lithography and process environment, which may also be preliminary. The designer must determine the critical locations in the layout.
There have been several publications in related areas. Recent approaches already proposed methods to statistically explore the relationship for CD, exposure and focus, (Shoji Mimotogi et al., “Statistical method for influence of exposure and focus error on CD Variation”, SPIE 4346, 303-308, 2001) and additionally, for mask CD. (Sonny Y. Zinn et al., “Simulation of critical dimension error using Monte Carlo method and its experimental verification”, 46th International Conference on Electron, Ion and Photon Beam Technology and Nanofabrication, 2002.) Mask errors were also included into the classical process window analysis. (Staf Verhaegen et al., “Analysis of the impact of reticle CD variations on the available process windows for a 100 nm CMOS process”, SPIE 4889, 197-208, 2002.). On the layout side, there are several approaches for test patterns which give access to lithography parameters, some of them even suitable for production type masks. (Christopher Ausschnitt, “Distinguishing dose from defocus for in-line lithography control”, SPIE 3677, 140-147, 1999) Furthermore “flexible mask specifications” were introduced, where one component is the selection of “hot spot” patterns for mask quality assessment. (Shigeki Nojima et al., “Flexible mask specifications”, SPIE 4754, 33-42, 2002.)
From the early development phase up to the production phase, test patterns play a key role for microlithography. The roles for test patterns is to represent the design well and to cover the space of all process conditions, e.g. to investigate the full process window and all other process parameters. However, the prior-art test patterns do not address these roles sufficiently.
In the prior art, test patterns are designed by hand, and they explore the geometry domain. However, some of the critical parameters are not well defined at the early design stages. Therefore, there is a significant need for a more comprehensive method for definition of test patterns.